The problem of repairing defective bone is clearly a continuing one. Until relatively recently, the only practical solution was to immobilize broken bones and rely on nature to effect regrowth of skeletal tissue into an injury. Only with the advent of the possibility of surgery has it become possible to actually implant bone substitutes, not only to replace injured or diseased bone structures, but also to repair congenital or degenerative defects in the skeletal structure.
A wide range of materials has since been utilized, and elaborate designs have been disclosed for replacements of entire portions of bones, e.g., for hip joints (U.S. Pat. No. 3,820,167) and teeth (U.S. Pat. No. 4,186,486). Materials employed have included metals such as titanium (EPO Pub. No. 0071242, published 9 Feb. 1983; U.S. Pat. No. 3,918,100), ceramics such as aluminum oxide (U.S. Pat. No. 3,919,723), shaped and treated bone (U.S. Pat. No. 3,318,774), and various bone preparations such as, for example, bone dust compacted into flexible mats (U.S. Pat. No. 2,621,145).
It has long been understood that skeletal structures have both inorganic and organic components. The inorganic component is a mineral, predominantly a form of calcium phosphate, hydroxyapatite. The organic component is chiefly composed of a single type of protein, collagen, which serves to impart a measure of resilience, thus preventing the structures from being unduly brittle. As skeletal tissue is alive, of course, additional metabolically active organic components must be included in the structure, and it is these bone cells and their active metabolites which are responsible for the naturally occurring healing and maintenance processes.
It has been determined that bone tissue repair occurs by one of two alternative mechanisms, or a combination of both. These mechanisms are referred to as conductive repair and inductive repair. In conductive repair, cells which are already committed to their character as bone cells (osteoprogenitor cells) move into the space of the defect from adjacent bone, and form bone directly. No special factors (other than non-specific nutrients) are required. In inductive repair, however, this process is preceded by conversion of previously uncommitted multipotential cells into osteoprogenitor cells which first form cartilage that calcifies and degenerates and is replaced by bone.
For either conductive or inductive repair, it is required that the living tissue or the host provide the ultimate skeletal structure. Thus the implant which mediates these processes serves not as a substitute for the defective or removed bone, but rather as a matrix support for active replacement of the missing tissue.
Accordingly, attempts have been made to devise implants for defective skeletal tissue or lesions in bones and teeth, which implants are intended precisely for this purpose. These implants do not attempt to mimic the composition of the missing bone, but rather serve as a structural support and guiding matrix for encroaching bone deposits derived from the adjacent fresh bone. These supports may provide only matrix support functions, i.e., mediate conductive repair, or they may, in addition, include factors which might mediate inductive repair such as by stimulating the differentiation of uncommitted cells to osteoprogenitor cells by providing what are currently known as "osteogenesis factors" (OF) or "bone morphogenic proteins" (BMP).
Because collagen is already a familiar material to the metabolically viable cells associated with bone growth, attempts have been made to use implants which are composed predominantly of collagen for both inductive and conductive repair.
Since the major components of bone from a quantitative standpoint are collagen and ceramic, various reconstituted implant compositions have been prepared using mixtures of ceramic materials and collagen. See, for example, U.S. Pat. No. 3,443,261; Hayashi, K., et al., Arc Orthop Traumat Surg 99:265 (1980); and U.S. Pat. No. 4,314,380.
In many collagen bone repair preparations it is thus necessary to initially mix a collagen dispersion or solution with a ceramic or mineral material. Additional factors as outlined above may be added to effect inductive rather than solely conductive repair.
Known mixing procedures, however, are not completely satisfactory. Typically a collagen dispersion is mixed with particulate ceramic material in a dish or other suitable container using an implement such as a spatula, rod, or the like. The process is frequently messy and time-consuming and, further, can result in problematic non-uniformity of the resulting paste. Thus, it is desirable to provide a mixing device which obviates these problems.
Several syringe-type mixing devices are known in the art. U.S. Pat. Nos. 4,254,768 to Ty and 4,538,920 to Drake each show a multiple barrel syringe device in which two materials are mixed just prior to injection. Similarly, U.S. Pat. No. 4,424,057 to House shows a "wet-dry" syringe which upon injection dissolves a solid component in a liquid component contained in a separate, internal vial. U.S. Pat. No. 4,496,344 to Kamstra shows a compartmental syringe in which two fluids are mixed upon injection. While several of these prior syringe-type mixing devices disclose dissolution of a solid in a fluid just prior to or upon injection, none shows a method for mixing a fluid or semi-fluid with a particulate solid in order to form a paste, nor is the specific preparation of bone repair compositions suggested.